Method for directing cellular migration patterns on a biological tissue

ABSTRACT

A contoured biological tissue for a bioprostheses, such as a cardiac/vascular patch or a bioprosthetic heart valve, and methods of contouring the tissue, are described. A predetermined pattern is provided on the tissue, comprising a plurality of ridges or depressions that are configured to facilitate cellular migration in a first direction and discourage cellular migration in a second direction. The biological tissue can be used in connection with a bioprosthetic heart valve comprising a biological tissue leaflet structure coupled to a supporting frame.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/495,621, filed Sep. 24, 2014, now, U.S. Pat. No. 10,959,839, whichclaims the benefit of U.S. Patent Application No. 61/888,080, filed Oct.8, 2013, the entire disclosures all of which are incorporated byreference for all purposes.

FIELD OF THE INVENTION

The invention relates to a method of treating biological tissue and,more particularly, to a method for imparting topographical changes ontoa surface of a bioprosthetic tissue to direct or control patterns ofcellular migration and tissue formation on the tissue.

BACKGROUND

Biological tissue, such as animal pericardium (e.g., bovine, porcine),has come into common use in connection with medical devices andimplantable bioprostheses, such as bioprosthetic heart valves andvascular patches. The use of biological tissue in implantedbioprostheses, however, is not without its complications. Once implantedin the body, the biological tissue may become calcified or may stimulatethe formation of scar or other types of tissue, such as pannus. Thebiological tissue may also stimulate cells to migrate into and integratethe implanted biological tissue into the host body.

One of the more serious complications associated with the implantationof prosthetic heart valves incorporating biological tissue isobstructive valve failure caused by pannus formation. Pannus is amembrane of granulation tissue, rich in fibroblasts, that forms inresponse to healing. The body may produce pannus where the native valvehas been removed and a prosthetic heart valve has been implanted.

In many cases, pannus growth does not encroach the valve orifice orchamber space, but occasionally the hanging edges can hit or obstruct aleaflet. When pannus overgrowth interferes with valve functioning,surgery is the only option to remove the pannus overgrowth. There is noreliable way of predicting whether a particular patient will be more orless susceptible to pannus overgrowth.

Pannus overgrowth is estimated to be the cause of obstructive heartvalve failure in a significant number of cases. In the aortic position,pannus formation occurs mainly on the inflow or ventricular side, whilein the mitral position, it occurs both on the atrial and ventricularsides. In order to prevent tissue ingrowth into the clearance ofleaflets, prosthetic valves have been designed with longer housingcylinders with the goal of creating an ingrowth barrier. Such valvedesigns, however, are often impractical, infeasible or undesirable forminimally-invasive or percutaneously-deliverable heart valves, as animportant design goal for such valves is to reduce the delivery profileas much as practically possible. Thus, the inclusion of additionalfeatures or structures which add to the material bulk of the valves isgenerally avoided.

What is therefore needed are bioprosthetic heart valves which controlpannus formation or which reduce or eliminate the likelihood of pannusovergrowth that interferes with proper valve functioning.

BRIEF SUMMARY

A contoured biological tissue for a bioprostheses, such as acardiac/vascular patch or a bioprosthetic heart valve, and methods ofpreparing the contoured tissue, are described herein. A predeterminedpattern is provided on the tissue, comprising a plurality of ridges ordepressions that are configured to facilitate cellular migration in afirst direction and discourage cellular migration in a second direction.The biological tissue can be used in connection with a bioprostheticheart valve comprising a biological tissue leaflet structure coupled toa supporting frame.

In one embodiment, a method for manufacturing a bioprosthetic heartvalve is described. The method comprises contouring a portion of abiological tissue valve leaflet with a predetermined pattern. Thepredetermined pattern comprises a plurality of ridges or depressionsoriented in the same or different directions. Adjacent ridges ordepressions are at least 10 microns apart to prevent or at least impedethe migration of fibroblasts transversely across the ridges ordepressions. The contouring can be performed with a laser, preferably afemtosecond laser.

The biological tissue can be at least partially crosslinked, and/or atleast partially dehydrated, such as with a glycerin-based treatmentsolution, before the contouring. The biological tissue can be treatedwith a capping agent after the crosslinking, after the contouring, orboth.

The method can further comprise packaging the bioprosthetic heart valvein a package that does not contain a liquid storage solution in contactwith the bioprosthetic heart valve.

In another embodiment, a contoured bioprosthetic heart valve isdescribed. The contoured bioprosthetic heart valve comprises one or aplurality of leaflets formed from a biological tissue and a contouredpattern is provided on the leaflets. The contoured pattern comprises aplurality of ridges or depressions, the distance between adjacent ridgesor depressions being at least 10 microns. The plurality of ridges ordepressions can comprise a first set of parallel depressions or ridges,and can further comprise a second set of parallel ridges or groovestransversing the first set of parallel ridges or grooves at an angle.

The bioprosthetic heart valve can further comprise sutures coupling theone or more leaflets to a support structure or a skirt. Thepredetermined pattern can be disposed on the leaflets adjacent to atleast a portion of the sutures. The one or plurality of heart valveleaflets can each comprise a straight free edge and an arcuate cusp edgeand the contoured pattern is provided substantially along the arcuatecusp edge.

In a further embodiment, a contoured bioprosthetic heart valve forimplantation within an arterial wall of a patient is described. Thecontoured bioprosthetic heart valve comprises a biological tissueleaflet structure coupled to a supporting frame. The biological tissueleaflet structure has a circumferential outer peripheral surface facingthe arterial wall. A contoured pattern is provided around thecircumferential outer peripheral surface. The contoured patterncomprises a plurality of ridges or depressions, spaced and/or sized toprevent or impede cellular migration across the ridges or depressions.The adjacent ridges or depressions are preferably at least 10 micronsapart. The contoured pattern can be provided along an entire length ofsutures coupling the biological tissue leaflet structure to thesupporting frame.

In yet a further embodiment, a contoured bioprosthetic heart valve isdescribed. The contoured bioprosthetic heart valve comprises one or aplurality of leaflets formed from a biological tissue. The leaflets eachcomprise a straight free edge and an arcuate cusp edge. A contouredpattern is provided on the leaflets. The contoured pattern comprises aplurality of ridges or depressions extending radially from the straightfree edge to the arcuate cusp edge. Cellular migration is promoted alongthe ridges or depressions.

In still another embodiment, a method for treating a biological tissueis described. The method comprises providing a tissue having a planarsurface, treating the tissue to reduce surface irregularities, andcontouring the tissue surface with a predetermined pattern comprisingone or a plurality of ridges or depressions. The ridges and depressionsare provided to encourage cellular migration in a first direction anddiscourage cellular migration in a second direction.

The tissue can be reduced in thickness to a range of about 250-500microns by compressing the tissue or by removing material from thetissue. The contouring can be performed by a laser, preferably afemtosecond laser. The tissue can be at least partially dried ordehydrated before the contouring, such as by using a glycerin-basedtreatment solution. The tissue can then be packaged in a package thatdoes not contain a liquid storage solution in contact with thebiological tissue.

Yet another embodiment comprises a biological tissue for implantationhaving a surface contoured with a predetermined pattern comprising atleast one ridge or depression configured to encourage cellular migrationin a first direction and discourage cellular migration in a seconddirection.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative embodiments of the present disclosure are described hereinwith reference to the accompanying drawings, in which:

FIG. 1A is a plan view of a bioprosthetic heart valve leaflet having aplurality of parallel grooves.

FIG. 1B is a plan view of a bioprosthetic heart valve leaflet having aplurality of parallel grooves disposed only along an area adjacent thecusp edge.

FIG. 2 is a plan view of a bioprosthetic heart valve leaflet having agrid pattern disposed along an area adjacent the cusp edge.

FIG. 3 is a plan view of a bioprosthetic heart valve leaflet having aradial pattern designed to encourage cellular migration on the leafletsurface.

FIG. 4 is a cross-sectional view along 4-4 of FIG. 1B showing thespacing and depth of the respective grooves.

FIG. 5 is a perspective view of a representative embodiment of aprosthetic heart valve that can be made with a biological tissue; and

FIG. 6 is a bottom perspective view of a valve leaflet structureconnected to a reinforcing skirt so as to form a leaflet assembly.

Like numerals refer to like parts throughout the several views of thedrawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Specific, non-limiting embodiments of the present invention will now bedescribed with reference to the drawings. It should be understood thatsuch embodiments are by way of example only and merely illustrative ofbut a small number of embodiments within the scope of the presentinvention. Various changes and modifications obvious to one skilled inthe art to which the present invention pertains are deemed to be withinthe spirit, scope and contemplation of the present invention as furtherdefined in the appended claims.

Described herein are methods for contouring a biological tissue with apredetermined pattern which are aimed at directing cellular migration ina predetermined pattern across a surface of a bioprosthetic implant.Such implants include heart valve leaflets and patches made of abiological tissue, such as a bovine, equine or porcine pericardium. In apreferred embodiment, the biological tissue is treated such that thetissue is thinned (by compression or by removal of tissue material)and/or the tissue surface is smoothed (by compression and/orcrosslinking), as described in U.S. Pub. No. 2011/0238167, publishedSep. 29, 2011, and U.S. Pat. No. 7,141,064, issued Nov. 28, 2006, theentire contents of each of which are incorporated herein by reference intheir entireties. In a preferred embodiment, the biological tissue isthinned to a thickness of about 100-800 microns, preferably about250-500 microns and most preferably about 100-300 microns.

FIGS. 1A-1B depict a bioprosthetic heart valve leaflet 100A having aplurality of parallel grooves 120. The leaflet 100A is depicted hereinas comprising an arcuate cusp edge 104, a generally straight free edge102 opposite the cusp edge 104, and a pair of oppositely-directed tabs106 at either end of the free edge 102. A plurality of depressions,e.g., grooves, or raised ridges 120 is provided on one side of theleaflet surface and is patterned to follow the shape of the arcuate cuspedge 104. As shown in FIG. 1A, the parallel grooves 120 can be providedon substantially the entire surface of the leaflet 100A or, as shown inFIG. 1B, the parallel grooves can be provided only along the areaadjacent the cusp edge 104 of the leaflet 100B. The patterns of parallelgrooves 120 depicted in FIGS. 1A-1B are provided to discourage cellularmigration in a transverse direction originating from the periphery orcentral region of the leaflet towards the central region or periphery ofthe leaflet, respectively.

As used herein, “parallel” refers to two paths that are substantiallyidentical but are offset so as to never intersect. Accordingly,“parallel” is broadly understood to encompass lines, curves and wavylines that follow the same path but are offset from one another by aconstant or variable distance. In a preferred embodiment, adjacent pathsare offset by a constant distance.

As used herein, “transversely” refers to a path that intersects thecontoured pattern, e.g., grooves or ridges, and is not limited to aparticular angle at which the path intersects the contoured pattern.

Certain cells, such as fibroblasts, are believed to have a significantrole in the formation and overgrowth of pannus on bioprosthetic heartvalves. Fibroblasts are a class of cells that synthesize theextracellular matrix and collagen, the structural framework for animaltissues. Fibroblasts play a critical role in wound healing. Implantationof bioprosthetic heart valves at a valve annulus stimulates fibroblaststo migrate and form a pannus around the heart valve and, morespecifically, in the areas surrounding the suture lines of abioprosthetic heart valve. It is therefore believed that directing themigration of cells (e.g., fibroblasts) responsible for pannus formationwill similarly help control the extent and location of pannus growth.

FIG. 5 depicts an embodiment of a bioprosthetic heart valve 50 asgenerally comprising a structural frame or stent 52, a flexible leafletstructure 54 supported by the frame 52 and a flexible skirt 56 securedto the outer surface of the leaflet structure 54. The valve 50 can beimplanted in the annulus of the native aortic valve, but also can beadapted to be implanted in other native valves of the heart or invarious other ducts or orifices of the body. Valve 50 has an inflow end60 and an outflow end 62. Valve 50 and frame 52 are configured to beradially collapsible to a collapsed or crimped state for introductioninto the body on a delivery catheter and radially expandable to anexpanded state for implanting the valve at a desired location in thebody, such as the native aortic valve.

Leaflet structure 54 desirably comprises three separate connectedleaflets 70, which are individually depicted in FIGS. 1-3. Theindividual leaflets can be arranged together to coapt in a tricuspidarrangement, as best shown in FIGS. 5 and 6. The leaflets 70 attach toone another at their adjacent sides to form commissures 80 of theleaflet structure 54. The leaflets are made of biological tissue,preferably bovine or porcine pericardium, and have a thickness in therange of about 100 to about 800 microns. The curved cusp edge forms asingle scallop in the leaflet structure 54 and when secured to two otherleaflets 70 to form the leaflet structure 54, the curved cusp edgescollectively form a scallop-shaped lower edge of the leaflet structure.A suture line 84 visible on the exterior of the skirt 56 tracks thescalloped shape of the leaflet structure 54. By forming the leafletswith this scalloped geometry, stresses on the leaflets are reduced,which in turn improves durability of the valve 50.

The skirt 56 can be formed, for example, of polyethylene terephthalate(PET) or pericardium, preferably bovine pericardium, ribbon. The leafletstructure 54 attaches to the skirt via a thin PET reinforcing strip 88or sleeve. The leaflet structure 54 is sandwiched between skirt 56 andthe reinforcing strip 88. The suture 84, which secures the reinforcingstrip and the leaflet structure 54 to skirt 56 can be any suitablesuture, and desirably tracks the curvature of the bottom edge of theleaflet structure 54 as seen on the exterior of the skirt 56 in FIG. 5.The skirt 56 and leaflet structure 54 preferably reside inside the frame52.

It is believed that pannus growth on bioprosthetic heart valvestypically occurs along the suture lines. Thus, with reference to thevalve 50 depicted in FIGS. 5 and 6, pannus growth would most likelyoriginate along the suture line 84 which corresponds to the cusp edge104 of the leaflets 100A and 100B in FIGS. 1A and 1B, typically alongthe outwardly facing external surface or the circumferential outerperipheral surface of the valve 50 that is in contact with the arterialwall or body lumen. Thus, disposing a plurality of grooves or ridges 120in a pattern that follows the suture line 84 will discourage or reducethe extent to which pannus growth occurs in a transverse direction tothe suture line 84 and reduce the extent to which pannus overgrowth willreach the inflow or outflow orifices or otherwise interfere with thecoaptation of the leaflets.

In a preferred embodiment, adjacent grooves or ridges 120 are bothdimensioned and offset at a distance that is greater than the averagesize of a fibroblast or other cell associated with pannus formation.FIG. 4 is a cross-sectional view along 4-4 of FIG. 1B depicting thespacing B between adjacent grooves and the width A and depth C of eachgroove 120. Fibroblasts are believed to have a size in the range of 10to 40 microns and more typically from 10 to 20 microns. Thus, in apreferred embodiment, adjacent grooves 120 are offset at a distance B ofat least 10 microns, preferably at least 20 microns, more preferably atleast 30 microns and most preferably at least 40 microns.

Similarly, one or both of the width A and depth C of the grooves 120 aredimensioned to be at least as large as, if not larger than, the averagesize of a fibroblast or other cell associated with pannus formation. Ina preferred embodiment, one or both of the width A and depth C of eachindividual groove 120 is at least 10 microns, preferably at least 20microns, more preferably at least 30 microns and most preferably atleast 40 microns.

It is understood that where the leaflets are made from a biologicaltissue that has been compressed or thinned, that the depth C of thegrooves 120 is selected so as to not compromise the strength of thetissue leaflets. In a preferred embodiment, the grooves 120 are providedon only the one side of the leaflet that faces the arterial or cardiacwall when the valve is implanted. The depth C of the grooves 120preferably does not exceed 25%, preferably 10% and more preferably 5%and most preferably 2% of the average thickness of the leaflet. Thus,for example, for a biological tissue leaflet having a thickness of 250microns, the depth C of the grooves preferably does not exceed 62.5microns, preferably 25 microns, more preferably 12.5 microns, and mostpreferably 5 microns. In the event that a width of less than 5 micronsis selected, it is understood that at least one of the other parameters,e.g., the distance B between grooves 120 or the width A of the grooves,is preferably selected to exceed the average cell size of at least 10microns. The values for A, B and C are selected to sequester or trap thefibroblasts or cells within grooves and/or between adjacent grooves 120.In embodiments where ridges are provided instead of grooves, theconsideration with respect to selecting the distance between ridges andheight of individual ridges is analogous to the distance between groovesand the depth of the grooves.

FIG. 2 depicts a bioprosthetic heart valve leaflet 200 having a gridpattern 210 disposed along an area adjacent the cusp edge 204 oppositethe free edge 202. Here, the grid pattern is provided by two sets ofintersecting parallel grooves or ridges which define a plurality ofsquare-shaped cells. In embodiments where grooves are employed, the sameconsiderations as to the distance between adjacent grooves and the widthand depth of the grooves as discussed in relation to FIGS. 1A and 1Bapply. In embodiments where ridges are employed, however, it is thedimension of the individual cells formed by the intersecting ridges thatare operative in preventing cellular migration. The two sets ofintersecting parallel ridges need not be at right angles to one anotherso long as the individual cells themselves are sized or dimensioned tobe larger than the average size of a fibroblast or other cell associatedwith pannus formation. Thus, in a preferred embodiment, the individualcells formed in the tissue have at least a width and a length of atleast 10 microns, preferably at least 20 microns, more preferably atleast 30 microns and most preferably at least 40 microns.

In certain instances, it is desirable to facilitate or encouragecellular growth across a biological tissue surface. In such instances,FIG. 3 depicts a bioprosthetic heart valve leaflet 300 in which radialgrooves 310 are provided as originating from a point 312 on the freeedge 302 of the leaflet and extending to the arcuate cusp edge 304.While not depicted in FIG. 3, the radial grooves 310 can also extendinto the tab portions 306 in a similar manner. The radial grooves 310function to direct the cellular migration along their length. Becausecellular interaction is desired, the distance between adjacent groovescan be no larger than the average size of fibroblasts or other cellsassociated with pannus formation as measured from the cusp edge. Thus ina preferred embodiment, this distance is less than 40 microns,preferably less than 30 microns, more preferably less than 20 micronsand most preferably less than 10 microns. Smaller distances betweenradial grooves 310 will permit migration transversely across the radialgrooves 310.

While the contouring of the biological tissue has been described anddepicted herein with respect to heart valve leaflets, it is understoodthat such contouring can be performed on any biological tissue that isintended for implantation in the body. For example, the contouring canbe provided in connection with biological tissue patches which are usedfor repair in cardiac and vascular reconstruction, soft tissuedeficiency repair, valve leaflet repair, carotid repair, closure ofpericardial defects, and suture line reinforcement during generalsurgical procedures. The dimensions and the location of the contouringwould depend upon the intended result: to stimulate cellular migrationalong a specified area of the tissue or to deter cellular migration andthus tissue formation in specified areas of the tissue. Thus, asexplained above, a groove being dimensioned with one or both of a widthand depth that is greater than the average diameter of a cell (e.g., 10,20, 30, or 40 microns) would likely trap the cells, prevent cellularmigration transversely across the groove and thus hinder the cells fromforming a network necessary to create unwanted tissue formation outsideof the groove. Similarly, a ridge can be configured to have a heightthat effectively prevents cellular migration across the ridge and thusaccomplish the same result.

The contouring of the biological tissue valve leaflets to producegrooves or ridges is preferably performed by laser ablation. In oneembodiment, the laser is a femtosecond laser. In another embodiment, thelaser includes a dual axis scanning lens, 2× beam expansion, 1550 nmwavelength, 31.5 μJ pulse energy on target; 1.6 W average power, 50 Hzrepetition rate, 650 fs pulse width (ref); 30 μm laser spot size,elliptical polarization, 112 mm focal length, 400 mm/s coarse millingspeed (20 μm fill spacing in cross hatch pattern), and 800 mm/s finemilling speed (20 μm fill spacing in cross hatch pattern).

In a preferred embodiment, the laser is coupled to a guiding device. Asubstantial amount of technology has been developed for guiding lasersand ablating tissue with great precision. Corneal ablation has beenwidely practiced and excimer lasers have become common. Reference ismade to U.S. Pat. No. 4,840,175, the disclosure of which is incorporatedherein by reference in its entirety. Recent work with mode lockinglasers having very short pulse lengths in picosecond and femtosecondranges with reduced heating is also suitable.

Milling machines for precisely guiding lasers are also available.Milling machines employing a laser having the above specifications asthe operative tool found to be useful for conveniently processingpericardium membranes have a 2-axis scanning laser head, tissue holdersto facilitate loading the work into the machine, an X-Y table toincrease working area of the laser and an automatic tissue holderloading mechanism. Mechanisms as described can be employed toselectively ablate a mounted pericardium membrane to generate patternsof different distances and dimensions as described herein. Furtherdescription of such mechanisms can be found, for example, in U.S. Pub.No. 2011/0238167, published Sep. 29, 2011, which is incorporated hereinby reference in its entirety.

The operation of the milling machine is automated according to inputdata defining the pattern and the coarseness of the ablation. Typicallysuch machines are arranged to control the depth of the ablation based onthe specific height of the surface being worked on. Thus in a preferredembodiment, the tissue is compressed or contoured to a substantiallyuniform thickness and height. Alternatively, a fixed reference can beused rather than the height of the tissue surface being cut.

Laser ablation to contour the tissue is understood to be advantaged ifperformed on a substantially dehydrated or dry tissue. This can beaccomplished by first fixing the tissue with a glycerin-based treatment.The tissue can first be cross-linked using glutaraldehyde or othersuitable fixative. The tissue can also be at least partially dehydratedor dried by other chemical or non-chemical means to permit storage ofthe contoured tissue in a non-fluid environment. Alternatively, thetissue can be at least partially dehydrated or dried prior tocontouring. Methods of treating tissue to at least partially dehydrateor dry the tissue, as compared to its native state, are disclosed inU.S. Pat. No. 8,007,992, issued Aug. 30, 2011 to Edwards Lifesciences,Corp. and U.S. Pat. No. 6,534,004, issued Mar. 18, 2003 to The ClevelandClinic Foundation, the entire contents of which are incorporated hereinby reference in their entireties. The tissue can then be mechanicallycompressed, cut into leaflets and contoured via laser ablation.

One contemplated sequence for contouring the biological tissue includesfirst cross-linking the tissue (e.g., bovine or porcine pericardium)with a glutaraldehyde-buffered solution. Next, the tissue can be heattreated using a process such as that disclosed in U.S. Pat. No.5,931,969 to Carpentier, issued Aug. 3, 1999, the disclosure of which isexpressly incorporated herein by reference. Subsequently, the thicknessof the tissue can be reduced by compression or by removing tissuematerial by laser or mechanical means such as by using a dermatome.Finally, the tissue can be treated with a capping and/or reducing agentto mitigate later in vivo calcification, which can also include treatingwith a glycerol/ethanol solution, as is described for example, in U.S.Pat. No. 7,972,376, issued Jul. 5, 2011 to Edwards Lifesciences Corp.,the entire contents of which are incorporated herein by reference in itsentirety. The tissue can also be at least partially dehydrated or driedby other chemical or non-chemical means to permit storage of thecontoured tissue in a non-fluid environment.

It should be understood that although cross-linking the tissue resultsin a somewhat easier to handle work piece, the contouring can occurprior to cross-linking as well. Likewise, bulk tissue sheet can becompressed and contoured first before or after fixing, or leaflets canfirst be cut from the bulk membrane which are then compressed andcontoured before or after fixing. Cross-linking the collagenous matrixprovides stability prior to implantation to retard degeneration.Further, the fixation process generally operates by blocking reactivemolecules on the surface of and within the donor tissue, therebyrendering it substantially non-antigenic and suitable for implantation.Fixing bioprosthetic tissue typically involves contacting the tissuewith a cross-linking agent, normally a solution. Exemplary fixingsolutions for bioprosthetic tissue such as bovine or porcine pericardiuminclude glutaraldehyde, formaldehyde, other aldehydes, EDC, polyethyleneglycol, etc. Other ways to fix tissue exist, including heating,irradiating, etc. The fixing step can help maintain the pericardium in aparticular three-dimensional form if undertaken after the membrane isotherwise prepared.

For prosthetic heart valve leaflets, the contoured leaflets are attachedto a surrounding heart valve support frame or other such components, andsterilized such as with ethylene oxide. After the tissue has beencontoured via laser ablation, calcification nucleation sites (e.g.,aldehydes and Schiff bases) can be exposed which creates a propensityfor calcification. Repeating the treatment with a capping agent (e.g.,ethanolamine) a reducing agent (e.g., sodium borohydride) and a collagenpreserving agent (e.g. glycerol) caps the nucleation sites and preservesthe collagen integrity following laser ablation. Furthermore, thisprocess will also allow the tissue to be stored in a non-liquid (i.e.,non-glutaraldehyde) environment. In other words, the process isespecially suitable for dry storage of the tissue.

The invention described and claimed herein is not to be limited in scopeby the specific preferred embodiments disclosed herein, as theseembodiments are intended as illustrations of several aspects of theinvention. Indeed, various modifications of the invention in addition tothose shown and described herein will become apparent to those skilledin the art from the foregoing description. Such modifications are alsointended to fall within the scope of the appended claims.

What is claimed is:
 1. A method for treating a biological tissue, themethod comprising: treating a biological tissue surface to reducesurface irregularities; and contouring the biological tissue surfacewith a predetermined pattern comprising one or a plurality of ridges ordepressions; wherein the one or plurality of ridges or depressions areprovided to encourage cellular migration in a first direction anddiscourage cellular migration in a second direction.
 2. The method ofclaim 1, wherein the biological tissue is pericardial tissue.
 3. Themethod of claim 2, wherein the pericardial tissue is selected from thegroup consisting of bovine pericardium, equine pericardium and porcinepericardium.
 4. The method of claim 1, wherein the treating comprisesthinning the biological tissue.
 5. The method of claim 4, wherein thebiological tissue is thinned to a thickness of about 100 to about 800microns.
 6. The method of claim 5, wherein the biological tissue isthinned to a thickness of about 250 to about 500 microns.
 7. The methodof claim 5, wherein the biological tissue is thinned to a thickness ofabout 100 to about 300 microns.
 8. The method of claim 4, wherein thethinning is performed by compressing the biological tissue, by removingmaterial from the biological tissue surface, or both.
 9. The method ofclaim 1, wherein the treating comprises smoothing the biological tissue.10. The method of claim 6, wherein the smoothing comprises compressingthe biological tissue, crosslinking the biological tissue or both. 11.The method of claim 1, wherein the contouring is performed by laser. 12.The method of claim 11, wherein the laser is a femtosecond laser. 13.The method of claim 11, further comprising substantially dehydrating ordrying the biological tissue before the contouring.
 14. The method ofclaim 11, further comprising treating the biological tissue with aglycerin-based treatment before the contouring.
 15. The method of claim11, further comprising treating the biological tissue with a fixative.16. The method of claim 15, wherein the fixative is glutaraldehyde. 17.A method for treating a biological tissue, the method comprising:cross-linking a biological tissue with a fixative; exposing thebiological tissue to a glycerin-based treatment; treating the biologicaltissue surface to reduce surface irregularities; contouring thebiological tissue surface with a predetermined pattern comprising one ora plurality of ridges or depressions; exposing the biological tissue toa capping agent, a reducing agent or both; wherein the one or pluralityof ridges or depressions are provided to encourage cellular migration ina first direction and discourage cellular migration in a seconddirection.
 18. The method of claim 17, wherein the fixative isglutaraldehyde.
 19. The method of claim 17, wherein the cross-linkingand the exposing the biological tissue with the glycerin-based treatmentis performed before the contouring.
 20. The method of claim 17, whereinthe exposing the tissue with the capping agent, the reducing agent orboth is performed after the treating and the contouring steps.